Hmmm... If it were a 1979 on, I would have said that the factory was forced to abandon the gravity furnaces due to changes in CSA surface temperature requirements. Since it is a 1973, this can't be the reason. The location of your furnace
is where some of the forced air furnaces were installed, by the factory. I suspect that the orignal owner just wanted more heat then the gravity furnaces could supply. He probably had this done as a custom option. Trillium
did several custom jobs.
BTW the fact that there is no vent for a gravity furnace
is not uncommon. If the factory furnace
was not selected, then no vent. It is possible that your forced air furnace is an aftermarket modification.
It looks good though. What is the manufacture, and model number of your furnace?